Trait dissection of maize kernel weight: Towards integrating hierarchical scales using a plant growth approach

Maize (Zea mays L.) yield is a function of the number harvested kernels per unit land area and the individual kernel weight (KW). Kernel weight and its development show a wide variability due to the genotype, the environment, the crop management, and all possible interactions. Commercial maize hybrids differ markedly in the patterns (rate and duration of kernel growth) behind differences in final KW. The same can be observed when public or elite proprietary maize inbred lines are analyzed. To progress in our understanding of KW variability, we reviewed and discussed current knowledge for analyzing kernel growth as an integrated system, modulated by processes linking different levels of organization (the different kernel tissues, the whole kernel, the plant, the canopy). Ideas on how to integrate this knowledge towards the development of a multi-hierarchical scale framework for predicting KW under different growth environments are currently needed, as they have high relevance for dissecting the genetic basis of kernel growth and maize yield definition at the canopy level. At the kernel and tissue level, we highlight the need of focusing and studying traits like assimilate movement into developing kernels, endosperm cell division, endosperm cell death, kernel internal homeostasis and kernel water relations. These specific processes need to be connected affecting the rate or duration of grain filling at the whole kernel level for studying kernel size variability. At the plant and canopy levels, the dynamic response of both kernel number and potential KW to plant growth around flowering appears critical for understanding KW variations, as maize final KW is highly associated with the potential KW determined during the first stages of grain filling. Focusing on the period around flowering in this species is important for yield improvement through both yield components, kernel number and size. At present it seems that maize potential yield can only be improved by modifying the sink capacity established at the end of the lag phase and increasing the source strength during grain filling so as to fulfill this potential.